For millennia, humans have sought physical relief, emotional healing, and even spiritual renewal by immersing themselves in water. Writer Lindsey Crittenden soaks it all in.

I slide up to my chin in hot, silky water. My jaw relaxes, my shoulders slacken, and my hips bob up. I’m floating without effort. At the other end of the flume—a long, rectangular tub of constantly flowing mineral water—I make out the shadows of two other bathers. No one speaks. …